The present invention relates to a surface inspection method and a surface inspection apparatus.
Conventionally, on a production line for a semiconductor substrates (semiconductor wafer), defects such as contaminant particles which have adhered to the surface of the substrate or scratches generated during working are inspected to monitor the dust generating situation of a production apparatus. For example, in the semiconductor substrate before circuit pattern forming, it is necessary to detect minute contaminant particles and defects as small as several nm or less on the surface. As for the inspection of the substrate surface, crystal defects existing in a shallow region near the substrate surface and surface roughness of the substrate surface also become subjects of the inspection, besides the above-described contaminant particles and defects. A technique for detecting minute defects on the surface of an object to be inspected such as a semiconductor substrate is described in, for example, U.S. Pat. No. 5,798,829. In other words, a semiconductor wafer or the like which is the inspection subject is mounted on an inspected object moving stage, and a partial region on the surface (illumination spot) is irradiated with illumination light generated by a laser light source. If in this state a contaminated particle which has adhered to the surface of the semiconductor wafer or a defect on the surface crosses the illumination spot, it generates scattered light. In the above-described conventional technique, the contaminated particle or defect is detected by catching the scattered light.
It is well known that if a contaminated particle or defect to be detected is sufficiently smaller than the illumination wavelength in the surface inspection apparatus described in the above-described conventional techniques intensity of light scattered by the contaminated particle or defect is proportional to approximately the sixth power of the particle size according to the Rayleigh scattering theory. It is also well known that the scattered light intensity is in inverse proportion to approximately the fourth power of the illumination wavelength according to the Rayleigh scattering theory. Until now, an Ar laser of 488 nm and a YAG second harmonic generation laser of 532 nm have been mainly used in the surface inspection apparatuses using the conventional techniques. However, higher sensitivity can be achieved by making the illumination length further shorter. By the way, in these laser light sources, there are a continuously oscillating CW laser and a pulse oscillating pulse laser depending upon the temporal form of emission. Although the Ar laser and the YAG second harmonic generation laser which have been mainly used are continuously oscillating type, there are a large number of pulse oscillating lasers in solid-state lasers having an oscillation wavelength in the ultraviolet region. An ultraviolet laser of 355 nm which is based on a YAG laser oscillating at 1064 nm and which utilizes its third harmonic (three times) generation is a representative one.
When using such a pulse laser in order to achieve the higher sensitivity, the following problem occurs. It is supposed that a pulse laser is used as the laser light source in the configuration of the surface inspection apparatus described in the conventional technique. In a typical pulse laser, for example, the repetition rate is in the range of approximately 50 to 180 MHz and the time width at half maximum of each emission pulse is in the range of approximately 10 to 30 ps. Except time delay caused by the length of the optical path, the scattered light at this time also has a temporal feature equivalent to the emission pulse of the laser light source, i.e., an equivalent repetition rate and an equivalent width at half maximum. A photomultiplier tube is typically used in a photodetector for detecting the scattered light. However, the time response characteristic of the photomultiplier tube is typically worse than the time width at half maximum of the pulse laser. As exemplified in FIG. 4, the time change waveform of the individual scattered light pulse in the output signal of the photomultiplier tube is largely distorted. Since the time response characteristic of the photomultiplier tube is approximately equal to or better than the repetition rate of the pulse laser, however, at least individual scattered light pulses are isolated and detected. An amplifier for amplifying the output signal of the photomultiplier tube needs to be narrow in bandwidth in order to reduce the shot noise component contained in the scattered light signal. On the other hand, a bandwidth needs to be broader than a definite width in order to find a detection position of a detected contaminated particle or defect accurately. In many cases, therefore, the bandwidth of the amplifier is typically set between DC and a frequency in the range of several MHz to several tens MHz. As a result of amplification in such a bandwidth, the scattered light pulses isolated and detected in the output signal of the photodetector are integrated to form a continuous waveform that nearly corresponds to an envelope of the scattered light pulses as exemplified in FIG. 4. However, the bandwidth of the amplifier is not wide enough to completely integrate and remove the original pulse trains. When an expanded output waveform of the amplifier is viewed, a ripple component caused by the original pulse train remains. It is apparent that the ripple component causes noise in the scattered light intensity signal. The detection sensitivity for contaminated particles or defects is thus lowered.